The present invention generally relates to x-ray imaging. In particular, the present invention relates to improved x-ray image data acquisition using a digital x-ray detector.
X-ray images may be used for many purposes. For instance, internal defects in an object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of internal structures in the object. The information gained from x-ray imaging has applications in many fields, including medicine and manufacturing.
X-ray imaging systems are commonly used to capture, for example, thoracic, cervical, spinal, cranial, and abdominal images that often include information necessary for a doctor to make an accurate diagnosis. X-ray imaging systems typically include an x-ray source and an x-ray detector. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against the x-ray detector as an x-ray technologist positions the x-ray detector and the x-ray source at an appropriate height. X-rays produced by the source travel through the patient's chest, and the x-ray detector then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected x-ray energy from the x-ray detector and prepares a corresponding diagnostic image on a display.
The x-ray detector may be an amorphous silicon flat panel detector, for example. Amorphous silicon is a type of silicon that is not crystalline in structure. Image pixels are formed from amorphous silicon photodiodes connected to switches on the flat panel. A scintillator is placed in front of the flat panel detector. The scintillator receives x-rays from an x-ray source and emits light in response to the x-rays absorbed. The light activates the photodiodes in the amorphous silicon flat panel detector. Readout electronics obtain pixel data from the photodiodes through data lines (columns) and scan lines (rows). Images may be formed from the pixel data. Images may be displayed in real time. Flat panel detectors may offer more detailed images than image intensifiers. Flat panel detectors may allow faster image acquisition than image intensifiers.
A solid state flat panel x-ray detector typically includes an array of picture elements (pixels) composed of Field Effect Transistors (FETs) and photodiodes. The FETs serve as switches, and the photodiodes are light detectors. The array of FETs and photodiodes may be composed of amorphous silicon. A compound such as Cesium Iodide (CsI) is deposited over the amorphous silicon. CsI absorbs x-rays and converts the x-rays to light. The light is then detected by the photodiodes. The photodiode acts as a capacitor and stores charge.
Initialization of the detector occurs prior to an x-ray exposure. During an initialization of the detector, the detector is “scrubbed” prior to an x-ray exposure. During scrubbing, each photodiode is reverse biased and charged to a known voltage. The detector is then exposed to x-rays which are absorbed by the CsI deposited on the detector. Light that is emitted by the CsI in proportion to x-ray flux causes the affected photodiodes to conduct, partially discharging the photodiode. After the conclusion of the x-ray exposure, a voltage on each photodiode is restored to an initial voltage. An amount of charge to restore the initial voltage on each affected photodiode is measured. The measured amount of charge becomes a measure of an x-ray dose integrated by a pixel during the length of the exposure.
The detector is read or scrubbed according to the array structure. That is, the detector is read on a scan line by scan line basis. A FET switch associated with each photodiode is used to control reading of photodiodes on a given scan line. Reading is performed whenever an image produced by the detector includes data, such as exposure data and/or offset data. Scrubbing occurs when data is to be discarded from the detector rather than stored or used to generate an image. Scrubbing is performed to maintain proper bias on the photodiodes during idle periods. Scrubbing may also be used to reduce effects of lag or incomplete charge restoration of the photodiodes, for example. Scrubbing restores charge to the photodiodes but the charge may not be measured. If the data is measured during scrubbing, the data may simply be discarded.
Switching elements in a solid state x-ray detector minimize a number of electrical contacts made to the detector. If no switching elements are present, at least one contact for each pixel is present in on the detector. Lack of switching elements may make the production of complex detectors prohibitive. Switching elements reduce the number of contacts to no more than the number of pixels along the perimeter of the detector array. The pixels in the interior of the array are “ganged” together along each axis of the detector array. An entire row of the array is controlled simultaneously when the scan line attached to the gates of the FETs of pixels on that row is activated. Each of the pixels in the row is connected to a separate data line through a switch. The switch is used by read out electronics to restore charge to the photodiode. As each row is activated, all of the pixels in the row have the charge restored to the respective photodiodes simultaneously by the read out electronics over the individual data lines. Each data line typically has a dedicated read out channel associated with the data line.
Image quality is an important criterion for a solid state x-ray detector design. In order to maximize image quality, noise generated by the read out electronics should be minimized. Furthermore, electronic noise affecting an image may be influenced by the resistance and capacitance of data lines reading out data from the detector array. To reduce the effect of data line resistance and capacitance on image noise, the data lines of the detector array may be split in half to reduce data line length. A shorter data line reduces the resistance and capacitance of each data line. Read out electronics may be added to two sides of the detector, rather than one side, to read the split data lines. However, the read out electronics with split data lines may operate at only half the speed of read out electronics with unsplit data lines to achieve the same detector read out rate.
Additionally, the detector electronics may be constructed in basic building blocks to provide modularity and ease of reconfiguration. Scan drivers, for example, may be modularized into a small assembly that incorporates drivers for 256 scan lines, for example. The read out channels may be modularized into a small assembly that would read and convert the signals from, for example, 256 data lines. The size, shape, architecture and pixel size of various solid state detectors applied to various imaging systems determine the arrangement and number of scan modules and data modules to be used.
A control board is used to read the detector. Programmable firmware may be used to adapt programmable control features of the control board for a particular detector. Additionally, a reference and regulation board (RRB) may be used with a detector to generate noise-sensitive supply and reference voltages (including a dynamic conversion reference) used by the scan and data modules to read data. The RRB also distributes control signals generated by the control board to the modules and collects data returned by the data modules. Typically, the RRB is designed specifically for a particular detector. An interface between the control board and the RRB may be implemented as a standard interface such that signals to different detectors are in a similar format.
Reading the detector may be accomplished as a pipelined process originating the data modules. As each scan line is activated (scan line N, for example), a data module read out channel acquires a signal (charge, for example) from the pixel on its data line that is activated by scan line N. While the data module is converting the charge acquired from scan line N, the data module may acquire a signal from scan line N+1. Then, while the data module is transferring or outputting the converted (digital) data from scan line N and converting the charge acquired from scan line N+1, charge may be acquired from scan line N+2. Data from each of the data modules is output to a bus based on a certain criteria, such as spatial or temporal order. Data flows from the data module through the RRB and then through the control board. In the control board, the data passes through a look up table (LUT) on the control board and then through programmable firmware, such as a Field Programmable Gate Array (FPGA) on the control board for formatting. The formatted data is then transmitted through an encoding device, such as a Fibre Channel encoding device, and through a data serialization device. Finally, the serialized electrical data is converted to a series of light pulses before the data leaves the control board through a fiber optic connection. A finite latency is maintained between when the data leaves a data module and when the data appears from the control board. The data may be re-registered at multiple points along the data path (adding one clock delay at each register, for example), and yet maintain a finite latency.
Currently, platform elements, such as the scan and data modules and control board hardware, are designed for the most demanding applications. Thus, less demanding or different applications suffer from inefficient performance. For example, vascular imaging is a demanding application for the data modules involving acquisition of very low levels of signal at high frame rates. For vascular imaging, data modules may consume more power than for other applications, such as chest radiography. Furthermore, since much of a detector is typically operated in a pipelined manner, increasing an acquisition frame rate for the detector involves optimizing multiple processes operating concurrently at comparable speeds. Thus, a simplified system and method for optimizing detector read out and acquisition frame rate would be highly desirable.
Additionally, data modules are currently designed using custom integrated circuits. Custom integrated circuits involve significant time to develop and to improve. Thus, there is a need for a system and method for achieving faster frame rates without relying on development of custom integrated circuits.
Thus, a need exists for improved x-ray image data acquisition using a digital x-ray detector.